In pixel based display systems, a display screen is divided into incremental components called pixels and the display of a color image is controlled by controlling the color of each pixel in the display. State of the art pixel display systems make use of a random access memory called a palette RAM as a look up table storing in its address locations different possible colors to be displayed in each of the pixels in the display device. Each location in the palette RAM will store three binary bytes representing a color. For example, the three bytes may represent the red, green and blue intensity components of the color. Alternatively, the three binary bytes may represent the color by values representing hue, brightness, and saturation or in the YIG color representing system. To cause a given pixel to be displayed with a selected color, the storage location in the palette RAM containing the color must be read out causing the stored bytes to be applied to digital-to-analog converters converting the color values to analog signals which become video signals. The palette RAM is part of a pixel decoder palette, also known as a RAMDAC, which receives pixel words representing palette RAM addresses in sequence, reads the color information out from the address locations selected by the received addresses, and converts the output signals read out from the palette RAM to the video signals to be applied to the video display device. In a state of the art system, the pixel decoder palette or RAMDAC is implemented in a single semiconductor chip.
In pixel based display systems, it is desirable to be able to switch the mode of operation of the pixel decoder palette from one mode of operation to another. For example, some pixel decoder palettes employ RAMS with each address location being operable to store 6-bit bytes representing color values. Other pixel decoder palettes employ palette RAMS each capable of storing 8-bit bytes. As a result, some of the pixel software, which is available to be used uses 6-bit color bytes and other pixel software makes use of 8-bit color bytes. Since the 6-bit pixel software is available to be used, it is desirable in systems having an 8-bit palette RAM to be able to switch the RAM to operate in response to either 6-bit data or 8-bit data.
In conventional pixel display systems, each pixel displayed must be one of the colors selected from the palette RAM. As a result, the edge of any object being displayed must always be displayed as occurring at the boundary between pixels. When an object edge is in the form of a diagonal line extending across the display screen, the object will appear distorted as a stair step or jagged edge. The eye is particularly sensitive to such stair step distortion and even when a large number of pixels are used to represent an object, the eye will perceive a diagonal line as a jagged edge. This artifact of pixel based displays is called aliasing.
U.S. Pat. No. 4,704,605 issued Nov. 3, 1987 to Steven D. Edelson, a coinventor of this application, discloses a system for smoothing the edges forming the boundaries between objects in pixel based displays to minimize the effect of aliasing. As described in this patent, each pixel through which a boundary or edge passes is controlled to display a color which effectively mixes the colors on each side of the boundary in amounts corresponding to where in the pixel the boundary occurs. By mixing the boundary or edge bridging pixels in this manner, the jaggedness of the edge as perceived by the viewer is substantially minimized. This method of reducing aliasing is referred to as continuous edge graphics. When a continuous edge graphics capability is provided, it is desirable to be able to switch between a conventional pixel display mode and a continuous edge graphics mode of operation.